Electron beam lithography is the most commonly used technique for performing direct writing—or maskless—lithography. It allows achieving a spatial resolution of a few tens of nanometers or less, and is particularly well suited for manufacturing photolithography masks.
FIG. 1 is a schematic illustration of an electron-beam lithography apparatus known from prior art. On this figure, reference 11 corresponds to a substrate—e.g. a silicon wafer or a glass or silica plate—onto which a pattern has to be transferred by direct writing lithography, reference 12 to a resist layer deposed on a surface of said substrate (the term “substrate” will be used indifferently to designate the bare substrate 11 or the ensemble 10 including the resist layer), reference 20 to an electron beam source, reference 21 to an electron beam generated by said source and impinging onto the resist layer 11, reference 30 to an actuation stage for translating the substrate 10 with respect to the electron beam 20, reference 40 to a computer or processor driving the electron beam source 20 and the actuation stage 30, and reference 41 to a computer memory device storing a program executed by said computer or processor 40. The electron beam source 20 and the actuation stage 30 cooperate for selectively exposing to the electron beam specific regions of the substrate, according to a predetermined pattern. Actually the spatial distribution of the energy deposed onto the substrate (the “dose”) does not accurately match the predetermined pattern; this is mainly due to the finite width of the electron beam and to the forward- and back-scattering resulting from the interactions of the electrons with the resist and the substrate (“proximity effects”).
Then, during a so-called development step, the exposed area (for positive resist) or the unexposed area (for negative resist) is selectively eliminated, so that the remaining resist approximately reproduces the predetermined pattern or its complement on the surface of the substrate. Afterwards, the portion of the surface of the substrate which is not covered by resist can be etched, and then the remaining resist eliminated. In different embodiments, the etching may be replaced by the implantation of a dopant, a deposition of matter etc.
Electron beam 21 may be a narrow circular beam, in which case the pattern is projected onto the resist point by point, using raster or vector scanning. In industrial applications, however, it is often preferred to use “shaped beams”, which are larger and typically have a rectangular or triangular section. In this case, before being transferred, the pattern is “fractured”—i.e. is decomposed into a plurality of elementary shapes which can be transferred by a single shot with a significant acceleration of the process.
When shaped beams are used, the number of shots—and therefore the number of elementary shapes which define the pattern—is the main factor determining the writing time, and therefore the cost of the process. Unfortunately, fractured patterns often comprise a significant number of elementary shapes, leading to long and expensive writing operations. Moreover, some of these elementary shapes may be smaller than the resolution of the direct writing process, and therefore impossible to reproduce accurately. This is particularly true for the writing of advanced photolithography masks, involving OPC (Optical Proximity Correction) treatments that may result in highly fragmented patterns. Several techniques have been developed in order to reduce the number of shots in direct writing using shaped particle or photon beams; a review is provided by the paper “Assessment and comparison of different approaches for mask write time reduction,” A. Elayat, T. Lin, S. F. Schulze, Proc. of SPIE, Vol. 8166, 816634-1-816634-13.
A first possibility consists in optimizing the fracturing step without modifying the pattern, but this only leads to a limited reduction of the shot count.
Better results may be obtained, but at a much greater computational cost, by allowing overlapping and non-abutting shots—i.e. by allowing that the fractured pattern does not correspond exactly to the non-fractured one (Model-Based Mask Data Preparation, or MB-MDP, see in particular G. S. Chua et al. “Optimization of mask shot count using MB-MDP and lithography simulation”, Proc. of SPIE, Vol. 8166, 816632-1-816632-11). This approach is complex to implement, and therefore slow and expensive.
“Jog alignment” is another shot-count reduction technique which consists in modifying the pattern before fracturing to remove misaligned jogs. A jog is a small (few nanometers) protruding or receding part in the edge of a pattern, usually created by the OPC. Misaligned jogs are jogs appearing on opposite edges of a feature but not directly facing each other. Said misaligned jogs would lead to the appearance, during fracturing, of small, sub-resolution elementary shapes, uselessly increasing the number of shots—see e.g. US 2009/0070732. This may result in a rather significant count reduction; however only a fraction of the sub-resolution features which could be removed harmlessly can be suppressed this way.
Use of L-shaped shots and multi-resolution writing (see the above-referenced paper by A. Elayat et al.) are also effective in reducing the shot count. However, the first requires a modification of the direct writing hardware, and the second of the writing process.
US 2014/245240 discloses a method wherein a first fracturing is performed and, if the fractured pattern is dimension-critical, a second fracturing is also performed.
US 2012/084740 discloses a fracturing method wherein the number of elementary shapes is reduced by using variable dose, different beam shape and by allowing overlapping of shots.
US 2012/329289 discloses, too, a method wherein the number of elementary shapes is reduced by allowing overlapping of shots.